Modern wire rod rolling lines are required to have significantly narrower tolerances with respect to the cross section of the rolled wire rod strand emerging from that line than can be obtained by manual correction of the parameters of the rolling mill line using classical wire rod rolling techniques. Not only must the cross sectional tolerances be significantly narrower, but deviations from the set point cross section must be held to a minimum over the entire length of the rolled wire rod. The tolerances of the wire rod following the finish rolling unit are determined by a number of parameters. Perhaps the greatest influence on the cross sectional tolerances is the cross section of the rolled wire rod strand entering the finishing unit or the group of mill stands forming the finishing unit and the entry temperature of the strand. Cross sectional variations and temperature variations of the strand entering the finishing unit in the past have been found to result in traction or tension fluctuations as the strand passes through the finishing unit and these fluctuations in turn give rise to substantial tolerance variations or fluctuations in the strand emerging from the finishing unit.
The entry cross section of the strand at the finishing unit depends, in turn, upon the traction applied in the preceding intermediate line or in any preceding mill stand or group of mill stands, among other parameters. The rolling conditions generally do not remain constant during the rolling of the rod but continuously tend to vary during the rolling process. These rolling conditions include not only the entry cross section parameter but parameters like temperature, longitudinal tension and others which contribute to cross sectional variations.
In order to achieve good tolerances, i.e. narrow tolerances, in spite of these continuously varying rolling conditions, the settings of any rolling units which contribute to the rolling conditions must be matched or adjusted closely. This is, however, extremely difficult in practice since with standard group drive of the rolls of the rolling unit, for example, the finishing unit, only the roll gaps can be adjusted and the drive speeds cannot be corrected. With previously known systems, considerable effort has been expended in attempts to eliminate the negative effects described above on the fabrication tolerances with the goal of obtaining a constant outlet cross section of the wire rod from the finishing rolling unit and to control, in spite of the aforementioned difficulties, the entry cross section of the wire rod strand into the finishing unit in a reliable manner.
Thus EP 200 396 B1 describes a system in which upstream of the finishing unit in the rolling direction, two sizing mill stands which are provided one after another and whose speeds are so controllable that with these stands a traction is applied which contributes a corresponding reduction in the wire rod cross section and makes it possible to feed the wire rod into the finishing unit with a relatively uniform cross section.